This document describes a new control strategy for dealing with failure of brake actuators in vehicles equipped with brake-by-wire systems and possibly with steer-by-wire systems. Brake-by-wire systems refer to any brake system in which brake actuators at each wheel can be controlled independently of the driver input and of each other. Steer-by-wire systems are steering systems in which the front (or rear) steering angle can be controlled independently of the driver, providing means of imparting a steer angle correction when necessary.
Algorithms for failure detection and identification in brake by wire systems have been developed. These algorithms either rely on sensor redundancy or use model-based techniques to detect and specify failure modes. It is therefore assumed in this document that when a failure occurs, it is sensed and reported to the control system. The main objective of the control algorithm during the failure mode is to redistribute the control tasks to the functioning actuators, so that the vehicle performance remains as close as possible to the desired performance despite the failure. This invention provides a means of accomplishing this task in an optimal way under all operating conditions.
During normal braking without failures, brake force distribution among four wheels is typically symmetric with respect to the longitudinal axis of vehicle symmetry. When one of the brake actuators fails it does not generate the desired force. This has two undesirable effects on vehicle dynamics: 1) vehicle deceleration is less than desired since the total braking force acting on the vehicle is reduced; and 2) brake force distribution becomes asymmetric, pulling the vehicle to the side as a result of unbalanced yaw moment acting on the vehicle. In order to maintain the desired level of deceleration, while minimizing the unbalanced yaw moment, the brake force distribution among the remaining three wheels must be modified. If the vehicle is equipped with steer by wire, an automatic steering correction may be generated in order to balance at least part of the yaw moment generated by asymmetric braking. The solution to this control allocation problem generally depends on the operating point of the vehicle, and the desired motion of the vehicle.
Consequently, a significant need exists for improving braking control after a failure of a brake-by-wire braking actuator.